If the key was a mutable object, its value could be changed. 如果鍵是可變對象，它的值可改變。
Classes in the Java class library are examples of immutable objects - they represent a single value that cannot change over the lifetime of the object. 類就是不變對象的示例它們表示在對象的生命期內無法更改的單個值。
滬公網安備 31011502000490號
電信業務許可證編號：滬B2-20190700
